In recent years, scanners are becoming more and more popular as computer peripheral equipment. Scanners are typically classified as sheetfed type and handheld type at the current stage. For both types of scanners, extra space near the computer to which the scanner is connected is needed for resting the scanner thereon or moving it therearound. In order to save working space, a scanner is designed to be mounted to a computer case, for example, at the location provided for a disk drive according to the present invention. When a scanner to be mounted to a computer case is designed, the current trend for requiring the minimization of the host computer should be simultaneously taken into consideration. In other words, the scanner had better occupy relatively small space from the host computer but be capable of scanning relatively large size of documents.